As an inverter unit according to the related art, for example, a unit described in Japanese Unexamined Patent Publication No. 2007-288858 is known. The inverter unit described in Japanese Unexamined Patent Publication No. 2007-288858 includes a switching element that converts a DC voltage into an AC voltage by switching the DC voltage and supplies three AC voltages to phases of a three-phase motor. The inverter unit includes a temperature detector that detects a temperature of the switching element and a line-to-line modulation controller that switches a modulation mode between a three-phase line-to-line modulation mode in which all supply voltages to three phases of the three-phase motor are switched in parallel and a two-phase line-to-line modulation mode in which only supply voltages to two phases of the three-phase motor are switched in parallel for each period of a predetermined electrical angle by controlling the switching element on the basis of the temperature detected by the temperature detector.
In the inverter unit according to the related art, the temperature of the switching element is detected and the three-phase line-to-line modulation mode is switched to the two-phase line-to-line modulation mode when the detected temperature is equal to or higher than a predetermined temperature. In the inverter unit, since a switching loss decreases when the modulation mode is switched, overheating of the switching element is prevented. However, when the modulation mode is switched from the three-phase line-to-line modulation mode to the two-phase line-to-line modulation mode, an output of the motor decreases.
An object of an aspect of the present invention is to provide an inverter unit that can protect a semiconductor element while securing an output of a motor.